This invention relates to a sensor for detecting inclination of an object, and more particularly, to a sensor having capacitive sectors within which the capacitance varies in accordance with the inclination of the sensor.
There are currently a number of leveling sensors in the art which may be used to measure the tilt of an object with respect to a vertical plane. In their simplest form, a carpenter's level having a transparent tube partially filled with fluid to leave a bubble which travels within the tube as the angle of tilt of the tube varies. Typically, this tube has two lines scribed onto it between which the bubble must rest to indicate that the sensor is level horizontally. When one side of the sensor tilts downward from a horizontal plane, the bubble moves in a direction opposite the downward motion outside of the scribed lines. Carpenter levels and the like indicate qualitative measurements of a vertical orientation of the sensing device or a deviation from vertical in a particular direction. Other than through visual inspection of the position of the bubble within the transparent tube, these levels do not indicate with specificity the angular measurement from a horizontal plane through which the sensor has tilted. For a number of applications, such qualitative measurements are sufficient, but for other applications, specificity of the angle of inclination is required.
A number of inclination sensing instruments based on the properties of the fluid and transparent tube level may be employed to determine deviations from the horizontal. In one such inclinometer, the tilt indication occurs through the use of gravity-type potentiometers. Electrical conductors are placed in a conical or circular tube containing fluid to a predetermined level. Two equal length segments of electrical conductor extend from the conducting fluid so that when the device is level, equal segments of the wire are submerged in the electrical conducting fluid. When the tube is tilted from a horizontal reference, the tube rotates, creating unequal length segments of electrical conductor within the resistance fluid. The variance in resistance between the wires is used to determine qualitatively the tilt of the sensor. This device is extremely sensitive to the resistance of the wire which varies in accordance with the temperature coefficient of resistivity of the wire and the temperature coefficient of expansion of the fluid.
In another variety of electrical inclination sensing instruments, variations in capacitance caused by variations in the displacement of fluid partially filling a sealed cavity indicate inclination. The fluid typically has a dielectric constant which is greater than that of the dielectric constant of the atmosphere of the cavity not occupied by fluid. This fluid, having a different dielectric constant, allows the variance of capacitance between sections of electrical conductors in the sensor. In a number of capacitance-type sensors, a circular plate is divided into a number of sections each having an electrode. As the position of the fluid varies covering different sections, the capacitance between each section varies. By measuring the differences in the section-to-section capacitance, the amount of tilt of the instrument may be determined. Other inclinometers employ two plates between which is established a capacitance by either a dielectric fluid (or an electrolytic fluid if the electrical conductors on the plate are covered by a dielectric coating). By determining the capacitance between the parallel plates, in accordance with the variance in fluid position due to the tilt of the sensor, a quantitative measurement of the horizontal tilt of the sensor may be determined.
In general, a number of tilt sensors exist which depend on varying the capacitance between sections comprising substantial portions of separate plate sections of the sensor according to the percentage of area covered by a fluid, either dielectric or electrolytic. They generally rely on a capacitance between conductive areas located on separate substrates as in multiple plates around a bubble in a vile or tube, multiple plates around a sphere, or multiple plates on the same axis. Such sensor assemblies tend to be difficult to construct, align, and are rather sensitive to rolling movements and to changes in temperature.
This invention is directed towards an improved inclination sensor in which electrically conductive elements are fabricated onto one or both flat surfaces of a plate shaped substrate. Fabrication onto a single flat substrate enables the manufacture of the electrically conductive elements directly onto the substrate using known methods such as etching in foil or other printed circuit board techniques. This provides a sensor having mechanical characteristics such as rigidity, coefficient of expansion, and sensitivity to temperature which are significantly related to the mechanical properties of the substrate assembly and less so to mechanical properties dictated by the overall sensor assembly operation, as is encountered in any number of parallel plate designs. The precision of the mechanical construction of the capacitive elements is a function of, for example, photolithographic etching processes for applying the pattern onto the substrate which can be done with great precision.
A plate comprising discrete, isolated sections each spanning a portion of the circumference of the sensor having a distinct electrically conductive regions is oriented in a substantially vertical plane. The plate has an axis which lies in a horizontal plane and which is substantially perpendicular to the plate so that the plate may tilt about the axis. The plate is encased within a sealed chamber occupied by a fluid having electrical properties having a higher specific gravity than the outer fluid which may be a gas which occupy the chamber. In one embodiment of the invention, the fluid is a dielectric fluid having a dielectric constant greater than that of a gas which occupies a second portion of the sealed chamber. The fluid flows in response to a change in the tilt of the sealed chamber and the plate. The capacitive fluid changes the capacitance between the distinct electrically conductive regions of each section of the plate. By determining the capacitance between the electrically conductive regions of each section, the tilt of the inclinometer may be determined.
In a second embodiment of this invention, the fluid is an electrolytic fluid, and the electrically conductive regions of each section are coated with a dielectric so that a capacitance may be established between the electrolytic fluid and the electrically conductive regions via the dielectric coating. With such a configuration, it is possible to vary the capacitance between the electrically conductive regions in a manner similar to that described above. When the inclinometer tilts about the axis, the position of the fluid varies in response to the change in tilt. By determining the capacitance between the electrically conductive regions of each section, the tilt of the sensor may be determined.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.